The Absence of Lymphotoxin-α, a Herpesvirus Entry Mediator (HVEM) Ligand, Affects Herpes Simplex Virus 1 Infection In Vivo Differently than the Absence of Other HVEM Cellular Ligands.

Previously, we reported that the absence of herpesvirus entry mediator (HVEM) decreases latency but not primary infection in ocularly infected mice. Recently, we reported that similar to the absence of HVEM, the absence of HVEM ligands (i.e., LIGHT, CD160, and B and T lymphocyte attenuator [BTLA]) also decreased latency but not primary infection. Similar to LIGHT, CD160, and BTLA, another member of tumor necrosis factor (TNF) superfamily, lymphotoxin-α (LTα), also interacts with HVEM. To determine whether LTα decreases latency in infected mice, we ocularly infected LTα-/- mice with latency-associated transcript-positive [LAT(+)] and LAT(-) viruses using similarly infected wild-type (WT) mice as controls. In contrast to WT C57BL/6 mice, LTα-/- mice were highly susceptible to ocular herpes simplex virus 1 (HSV-1) infection, independent of the presence or absence of LAT. Survival was partially restored by adoptive transfer of CD4+, CD8+, or total T cells. Infected LTα-/- mice had significantly higher corneal scarring than WT mice, and adoptive T cell transfer did not alter the severity of eye disease. In contrast to results in WT mice, the amount of latency was not affected by the absence of LAT. The amount of LAT RNA in LTα-/- mice infected with LAT(+) virus was similar to that in WT mice, and adoptive T cell transfer did not alter LAT RNA levels in LTα-/- infected mice. Increased latency in the absence of LTα correlated with upregulation of HVEM, LIGHT, CD160, and BTLA transcripts as well as with an increase in markers of T cell exhaustion. The results of our study suggest that LTα has antipathogenic and anti-inflammatory functions and may act to protect the host from infection.IMPORTANCE Recently, we evaluated the effects of HVEM and its ligands (LIGHT, CD160, and BTLA) on HSV-1 infectivity. However, the effect of LTα, another member of the TNF superfamily, on HSV-1 latency and eye disease is not known. Here, we demonstrate increased latency and corneal scarring in LTα-/- infected mice, independent of the presence of LAT. In addition, infected mice were highly susceptible to HSV-1 infection, and survival was partially but not significantly restored by adoptive T cell transfer. These results suggest that the absence of LTα affects HSV-1 infectivity differently than the absence of HVEM, LIGHT, CD160, and BTLA.

Inactivation of TNF/LT locus alters mouse metabolic response to concentrated ambient PM2.5.

BACKGROUND: Exposure to ambient fine particulate matter (PM2.5) is associated with increased cardiometabolic morbidity and mortality. This is widely believed to be attributable to PM2.5 exposure-induced pulmonary and subsequent systemic inflammation. Tumor necrosis factor alpha (TNFα), lymphotoxin α (LTα), and lymphotoxin ß (LTß) are three homologous pro-inflammatory cytokines, each with both unique and redundant activities in inflammation. Their role in PM2.5 exposure-induced inflammation and adverse cardiometabolic effects has to be determined. METHODS AND RESULTS: LTα/TNFα/LTß triple-knockout (TNF/LT KO) and wildtype (WT) mice were exposed to concentrated ambient PM2.5 (CAP) for 5 months. Lung pathological analysis revealed that TNF/LT deficiency reduced CAP exposure-induced pulmonary inflammation. However, glucose homeostasis assessments showed that TNF/LT deficiency significantly aggravated CAP exposure-induced glucose intolerance and insulin resistance. Consistent with glucose homeostasis assessments, CAP exposure significantly increased the body weight and adiposity of TNF/LT KO but not WT mice. In contrast to its body weight effects, CAP exposure reduced food intake of WT but not TNF/LT KO mice. On the other hand, CAP exposure induced marked fat droplet accumulation in brown adipose tissues of WT mice and significantly decreased their uncoupling protein 1 (UCP1) expression, and these effects were markedly exacerbated in TNF/LT KO mice. CONCLUSION: The present study suggests that TNF/LT deficiency influences PM2.5 exposure-induced response of energy metabolism through alterations in both food intake and energy expenditure.

Lymphotoxin alpha-deficient mice clear persistent rotavirus infection after local generation of mucosal IgA.

Rotavirus is a major cause of pediatric diarrheal illness worldwide. To explore the role of organized intestinal lymphoid tissues in infection by and immunity to rotavirus, lymphotoxin alpha-deficient (LTα(-/-)) mice that lack Peyer's patches and mesenteric lymph nodes were orally infected with murine rotavirus. Systemic rotavirus was cleared within 10 days in both LTα(-/-) and wild-type mice, and both strains developed early and sustained serum antirotavirus antibody responses. However, unlike wild-type mice, which resolved the intestinal infection within 10 days, LTα(-/-) mice shed fecal virus for approximately 50 days after inoculation. The resolution of fecal virus shedding occurred concurrently with induction of intestinal rotavirus-specific IgA in both mouse strains. Induction of intestinal rotavirus-specific IgA in LTα(-/-) mice correlated with the (late) appearance of IgA-producing plasma cells in the small intestine. This, together with the absence of rotavirus-specific serum IgA, implies that secretory rotavirus-specific IgA was produced locally. These findings indicate that serum IgG responses are insufficient and imply that local intestinal IgA responses are important for the clearance of rotavirus from intestinal tissues. Furthermore, they show that while LTα-dependent lymphoid tissues are important for the generation of IgA-producing B cells in the intestine, they are not absolutely required in the setting of rotavirus infection. Moreover, the induction of local IgA-producing B cell responses can occur late after infection and in an LTα-independent manner.

Lymphotoxin regulates commensal responses to enable diet-induced obesity.

Microbiota are essential for weight gain in mouse models of diet-induced obesity (DIO), but the pathways that cause the microbiota to induce weight gain are unknown. We report that mice deficient in lymphotoxin, a key molecule in gut immunity, were resistant to DIO. Ltbr(-/-) mice had different microbial community composition compared to their heterozygous littermates, including an overgrowth of segmented filamentous bacteria (SFB). Furthermore, cecal transplantation conferred leanness to germ-free recipients. Housing Ltbr(-/-) mice with their obese siblings rescued weight gain in Ltbr(-/-) mice, demonstrating the communicability of the obese phenotype. Ltbr(-/-) mice lacked interleukin 23 (IL-23) and IL-22, which can regulate SFB. Mice deficient in these pathways also resisted DIO, demonstrating that intact mucosal immunity guides diet-induced changes to the microbiota to enable obesity.

In vivo depletion of lymphotoxin-alpha expressing lymphocytes inhibits xenogeneic graft-versus-host-disease.

Graft-versus-host disease (GVHD) is a major barrier to successful allogeneic hematopoietic cell transplantation and is largely mediated by activated donor lymphocytes. Lymphotoxin (LT)-α is expressed by subsets of activated T and B cells, and studies in preclinical models demonstrated that targeted depletion of these cells with a mouse anti-LT-α monoclonal antibody (mAb) was efficacious in inhibiting inflammation and autoimmune disease. Here we demonstrate that LT-α is also upregulated on activated human donor lymphocytes in a xenogeneic model of GVHD and targeted depletion of these donor cells ameliorated GVHD. A depleting humanized anti-LT-α mAb, designated MLTA3698A, was generated that specifically binds to LT-α in both the soluble and membrane-bound forms, and elicits antibody-dependent cellular cytotoxicity (ADCC) activity in vitro. Using a human peripheral blood mononuclear cell transplanted SCID (Hu-SCID) mouse model of GVHD, the anti-human LT-α mAb specifically depleted activated LT-expressing human donor T and B cells, resulting in prolonged survival of the mice. A mutation in the Fc region, rendering the mAb incapable of mediating ADCC, abolished all in vitro and in vivo effects. These data support a role for using a depleting anti-LT-α antibody in treating immune diseases such as GVHD and autoimmune diseases.

Lymphotoxin-alpha contributes to lymphangiogenesis.

Lymphotoxin-α (LTα), lymphotoxin-ß (LTß), and tumor necrosis factor-α (TNFα) are inflammatory mediators that play crucial roles in lymphoid organ development. We demonstrate here that LTα also contributes to the function of lymphatic vessels and to lymphangiogenesis during inflammation. LTα(-/-) mice exhibited reduced lymph flow velocities and increased interstitial fluid pressure. Airways of LTß(-/-) mice infected with Mycoplasma pulmonis had significantly more lymphangiogenesis than wild type (WT) or LTα(-/-) mice, as did the skin draining immunization sites of LTß(-/-) mice. Macrophages, B cells, and T cells, known sources of LT and TNFα, were apparent in the skin surrounding the immunization sites as were LTα, LTß, and TNFα mRNAs. Ectopic expression of LTα led to the development of LYVE-1 and Prox1-positive lymphatic vessels within tertiary lymphoid organs (TLOs). Quantification of pancreatic lymphatic vessel density in RIPLTαLTß(-/-) and WT mice revealed that LTα was sufficient for inducing lymphangiogenesis and that LTß was not required for this process. Kidneys of inducible LTα transgenic mice developed lymphatic vessels before the appearance of obvious TLOs. These data indicate that LTα plays a significant role in lymphatic vessel function and in inflammation-associated lymphangiogenesis.

Initiation of acquired immunity in the lungs of mice lacking lymph nodes after infection with aerosolized Mycobacterium tuberculosis.

Recent evidence points to lung draining lymph nodes as the site that initiates the immune response in mice infected with aerosolized Mycobacterium tuberculosis. Here we expanded these studies and showed that infection of mice that lack lymph nodes with aerosolized M. tuberculosis results in a massive mononuclear cell infiltrate in the lungs within 14 days postinfection. This infiltration clearly resembles an expansion of the bronchus-associated lymphoid tissue. As expected, no bronchus-associated lymphoid tissue was observed in M. tuberculosis-infected wild-type control mice. Importantly, acquired specific immune response to M. tuberculosis antigens could be detected in lung lymphocytes harvested from mice lacking lymph nodes as early as 14 days postinfection. In addition, the bacterial burden in these mice was indistinguishable from that observed in wild-type C57BL/6 control mice. These results indicate that in the absence of lymph nodes, priming of the immune response occurs in the lung tissues after infection of mice with aerosolized M. tuberculosis and clearly illustrate the enormous plasticity of the immune system to develop resistance to foreign pathogens.

FTY720 suppresses the development of colitis in lymphoid-null mice by modulating the trafficking of colitogenic CD4+ T cells in bone marrow.

2-amino-2-(2-[4-octylphenyl]ethyl)-1,3-propanediol hydrochloride (FTY720) suppresses T-cell egress from LN, thereby preventing pathogenic T cells from migrating toward disease sites. However, little is known about whether FTY720 could control the trafficking of T cells without the presence of lymphoid tissues. Here we demonstrate that FTY720 treatment suppresses the recirculation of CD4(+) T cells in splenectomized (SPX) lymphotoxin-alpha(-/-) (LT-alpha(-/-)) mice that lack LN and spleen, as shown by peripheral blood (PB) lymphopenia in FTY720-treated SPX LT-alpha(-/-) mice. In a short-term transfer experiment, the cell number of transferred Ly5.1(+)CD4(+) T cells recovered from host FTY720-treated SPX LT-alpha(-/-) mice (Ly5.2(+)) was markedly decreased in PB, but conversely increased in BM. Notably, FTY720 treatment prevented the development of colitis that is otherwise induced in untreated SPX LT-alpha(-/-) x RAG-2(-/-) mice upon transfer of colitic lamina propria CD4(+) T cells. In such mice, the number of CD4(+) T cells in PB or lamina propria of FTY720-treated SPX LT-alpha(-/-) x RAG-2(-/-) recipients was significantly reduced, but that in the BM was significantly increased as compared with untreated control mice. Altogether, the present results indicate that FTY720 treatment may offer an additional role to direct trafficking of CD4(+) T cells in BM, resulting in the prevention of colitis.

Increased voluntary exercise in mice deficient for tumour necrosis factor-alpha and lymphotoxin-alpha.

BACKGROUND: The endogenous mediators playing a role in the sensing of fatigue and cessation of exercise are yet to be characterized. We hypothesized that proinflammatory cytokines, in particular tumour necrosis factor-alpha (TNFalpha) and lymphotoxin-alpha (LT) transmit signals leading to fatigue. MATERIALS AND METHODS: Mice were placed in a cage with a freely rotating exercise wheel and allowed to adapt for 24 h. The running distance was measured for two additional periods of 24 h. The effects of the administration of intravenous anti-TNF antibodies, intracerebral recombinant TNF, or intravenous lipopolysaccharide (LPS) were also determined. RESULTS: Compared to normal littermates, the voluntary daily running distance was 1.8-fold greater in mice with a disruption of the gene for TNFalpha, and 3-fold greater in mice with a gene disruption for both TNFalpha and LT. Intravenous administration of a monoclonal antibody against murine TNFalpha did not affect the running distance of wild-type mice, whereas administration of TNF intracerebrally reduced by 4-fold the voluntary running distance of the animals. This demonstrates that fatigue is mediated by TNFalpha expressed in the central nervous system (CNS) and not by increased peripheral TNFalpha concentrations. TNFalpha and LT are strong inducers of prostaglandins, but mice with disrupted prostaglandin or prostacyclin receptors exhibited running distances not significantly different from their wild-type littermates. Thus, signalling molecules other than prostaglandins mediate the effect of TNFalpha and LT on exercise capacity. CONCLUSIONS: Our finding that exercise capacity is controlled by TNFalpha is the first to define the endogenous mediators of fatigue, and may have important implications for diseases with impaired exercise tolerance.

Toll-like receptor-4 signaling and Kupffer cells play pivotal roles in the pathogenesis of non-alcoholic steatohepatitis.

BACKGROUND/AIMS: Studies in animal models and humans suggest a link between endotoxemia and non-alcoholic steatohepatitis. Since Kupffer cells are responsible for clearing endotoxin and are activated via endotoxin interaction with Toll-like receptor 4 (TLR-4), we examined the relationship between hepatic TLR-4 expression and Kupffer cell content during the genesis of steatohepatitis. METHODS: Male C57BL/6, C3H/HouJ and TLR-4 mutant C3H/HeJ mice were fed control or methionine/choline-deficient diet (MCDD). In one group of C57BL/6 mice, Kupffer cells were depleted by weekly intraperitoneal injections of clodronate liposomes. After 3 weeks, serum ALT activity and portal endotoxin levels were measured. Real-time PCR was used to examine mRNA expression of TLR-4, TLR-2, CD14, MD-2, TGFbeta, TNFalpha, CD36, PPAR-alpha, liver fatty acid binding protein (L-FABP) and collagen alpha1. RESULTS: We observed histological evidence typical of steatohepatitis, portal endotoxemia and enhanced TLR-4 expression in wild type mice fed MCDD. In contrast, injury and lipid accumulation markers were significantly lower in TLR-4 mutant mice. Destruction of Kupffer cells with clodronate liposomes blunted histological evidence of steatohepatitis and prevented increases in TLR-4 expression. CONCLUSIONS: These findings demonstrate the importance of TLR-4 signaling and underscore a direct link between TLR-4 and Kupffer cells in the pathogenesis of steatohepatitis.

Lymphotoxin beta receptor signaling is required for inflammatory lymphangiogenesis in the thyroid.

Infiltration of lymphocytes into the thyroid gland and formation of lymph node-like structures is a hallmark of Hashimoto's thyroiditis. Here we demonstrate that lymphatic vessels are present within these infiltrates. Mice overexpressing the chemokine CCL21 in the thyroid (TGCCL21 mice) developed similar lymphoid infiltrates and lymphatic vessels. TGCCL21 mice lacking mature T and B cells (RAGTGCCL21 mice) did not have cellular infiltrates or increased number of lymphatic vessels compared with controls. Transfer of CD3(+)CD4(+) T cells into RAGTGCCL21 mice promoted the development of LYVE-1(+)podoplanin(+)Prox-1(+) vessels in the thyroid. Genetic deletion of lymphotoxin beta receptor or lymphotoxin alpha abrogated development of lymphatic vessels in the inflamed areas in the thyroid but did not affect development of neighboring lymphatics. These results define a model for the study of inflammatory lymphangiogenesis in the thyroid and implicate lymphotoxin beta receptor signaling in this process.

Role of the GALT in scrapie agent neuroinvasion from the intestine.

Following oral exposure, some transmissible spongiform encephalopathy (TSE) agents accumulate first upon follicular dendritic cells (FDCs) in the GALT. Studies in mice have shown that this accumulation is obligatory for the efficient delivery of the TSE agent to the brain. However, which GALTs are crucial for disease pathogenesis is uncertain. Mice deficient in specific GALT components were used here to determine their separate involvement in scrapie agent neuroinvasion from the intestine. In the combined absence of the GALTs and FDCs (lymphotoxin (LT)alpha(-/-) mice and LTbeta(-/-) mice), scrapie agent transmission was blocked. When FDC maturation was induced in remaining lymphoid tissues, mice that lacked both Peyer's patches (PPs) and mesenteric lymph nodes (wild-type (WT)-->LTalpha(-/-) mice) or PPs alone (WT-->LTbeta(-/-) mice) remained refractory to disease, demonstrating an important role for the PPs. Although early scrapie agent accumulation also occurs within the mesenteric lymph nodes, their presence in WT-->LTbeta(-/-) mice did not restore disease susceptibility. We have also shown that isolated lymphoid follicles (ILFs) are important novel sites of TSE agent accumulation in the intestine. Mice that lacked PPs but contained numerous FDC-containing mature ILFs succumbed to scrapie at similar times to control mice. Because the formation and maturation status of ILFs is inducible and influenced by the gut flora, our data suggest that such factors could dramatically affect susceptibility to orally acquired TSE agents. In conclusion, these data demonstrate that following oral exposure TSE agent accumulation upon FDCs within lymphoid tissue within the intestine itself is critically required for efficient neuroinvasion.

Lymphotoxin alpha and tumour necrosis factor are not required for control of parasite growth, but differentially regulate cytokine production during Plasmodium chabaudi chabaudi AS infection.

Tumour necrosis factor (TNF) plays important roles in the pathogenesis of severe malaria, as well as in the generation of immune responses against malaria parasites. However, far less is known about the role of the closely related TNF family member lymphotoxin alpha (LTalpha) during malaria. We have used mice deficient in either TNF or LTalpha, as well as chimeric mice generated using donor bone marrow from these animals, to study the roles of these cytokines following Plasmodium chabaudi chabaudi AS infection. TNF and LTalpha were not required for the resolution of P. chabaudi chabaudi AS blood-stage infection. However, LTalpha, but not TNF, was necessary for early IFNgamma production and the regulation of IFNgamma production later in infection. A similar delay to that found for IFNgamma production was also observed for TNF production in LTalpha-deficient mice, compared with control mice. These results identify divergent roles for TNF and LTalpha in the regulation of host immune responses during P. chabaudi chabaudi AS infection.

Role of the draining lymph node in scrapie agent transmission from the skin.

Transmissible spongiform encephalopathies (TSEs) are neurodegenerative diseases that affect humans and animals. Diseases include scrapie in sheep and Creutzfeldt-Jakob disease in humans. Following peripheral exposure, TSE agents usually accumulate on follicular dendritic cells (FDCs) in lymphoid tissues before neuroinvasion. Studies in mice have shown that TSE exposure through scarified skin is an effective means of transmission. Following inoculation by this route TSE agent accumulation upon FDCs is likewise essential for the subsequent transmission of disease to the brain. However, which lymphoid tissues are crucial for TSE pathogenesis following inoculation via the skin was not known. Mice were therefore created that lacked the draining inguinal lymph node (ILN), but had functional FDCs in remaining lymphoid tissues such as the spleen. These mice were inoculated with the scrapie agent by skin scarification to allow the role of draining ILN in scrapie pathogenesis to be determined. We show that following inoculation with the scrapie agent by skin scarification, disease susceptibility was dramatically reduced in mice lacking the draining ILN. These data demonstrate that following inoculation by skin scarification, scrapie agent accumulation upon FDCs in the draining lymph node is critical for the efficient transmission of disease to the brain.

Naive recirculating B cells mature simultaneously in the spleen and bone marrow.

We have recently demonstrated that IgD(hi) B cells can occupy an extravascular perisinusoidal niche in the bone marrow in addition to the well-established follicular niche in conventional secondary lymphoid organs. The spleen has long been considered to be the site at which newly formed B lymphocytes mature into IgD(hi) naive recirculating B cells, but the existence of mutant mice that have selectively lost mature B cells in the bone marrow prompted an examination of B-cell maturation at this latter site. Following a single pulse of BrdU in intact mice, sequential labeling of more mature B-cell populations in the bone marrow suggested ongoing maturation at this site. Further evidence for B-cell maturation in the bone marrow was obtained from analyses of transitional B cells in splenectomized lymphotoxin alpha-deficient mice that lack all secondary lymphoid organs. In these mice, antibody-secreting cells recognizing multivalent antigens were also observed in the bone marrow following an intravenous microbial challenge. These data suggest that newly formed B cells mature into IgD(hi) B cells simultaneously in the spleen and the bone marrow and establish in a stringent manner that humoral immune responses can be initiated in situ in the bone marrow.

Function of CD4+CD3- cells in relation to B- and T-zone stroma in spleen.

Lymphocytes from lymphotoxin (LT) alpha-deficient mice, which lack segregation of their B- and T-cell areas, acquire normal organization following adoptive transfer into RAG-deficient recipients, identifying a non-B non-T cell in the segregation process. Here we show that a CD4+CD3- accessory cell is tightly associated with discrete VCAM-1-expressing stromal cells in B- and T-cell areas of the mouse spleen. CD4+CD3- cells express high levels of LTalpha, LTbeta, and tumor necrosis factor (TNF) alpha, which are the ligands for the LTbeta receptor and TNFR1 expressed by stromal cells. The expression of these ligands is functional, as transferring CD4+CD3- cells derived from either embryonic or adult tissues into LTalpha-deficient mice organizes B/T segregation and up-regulates CCL21 protein expression in areas where T cells are segregated from B cells. We propose that the function of CD4+CD3- cells is to form a link between primed CD4 T cells and the underlying stromal elements, creating distinct microenvironments in which they enable effector responses.

Immunohistochemical characterization of Fas (CD95) and Fas Ligand (FasL/CD95L) expression in the injured brain: relationship with neuronal cell death and inflammatory mediators.

Traumatic brain injury causes progressive tissue atrophy and consequent neurological dysfunction, resulting from neuronal cell death in both animal models and patients. Fas (CD95) and Fas ligand (FasL/CD95L) are important mediators of apoptosis. However, little is known about the relationship between Fas and FasL and neuronal cell death in mice lacking the genes for inflammatory cytokines. In the present study, double tumor necrosis factor/lymphotoxin-alpha knockout (-/-) and interleukin-6-/- mice were subjected to closed head injury (CHI) and sacrificed at 24 hours or 7 days post-injury. Consecutive brain sections were evaluated for Fas and FasL expression, in situ DNA fragmentation (terminal deoxynucleotidyl transferase-mediated dUTP-biotin nick end-labeling; TUNEL), morphologic characteristics of apoptotic cell death and leukocyte infiltration. A peak incidence of TUNEL positive cells was found in the injured cortex at 24 hours which remained slightly elevated at 7 days and coincided with maximum Fas expression. FasL was only moderately increased at 24 hours and showed maximum expression at 7 days. A few TUNEL positive cells were also found in the ipsilateral hippocampus at 24 hours. Apoptotic, TUNEL positive cells mostly co-localized with neurons and Fas and FasL immunoreactivity. The amount of accumulated polymorphonuclear leukocytes and CD11b positive cells was maximal in the injured hemispheres at 24 hours. We show strong evidence that Fas and FasL might be involved in neuronal apoptosis after CHI. Furthermore, Fas and FasL upregulation seems to be independent of neuroinflammation since no differences were found between cytokine-/- and wild-type mice.

Development of nephritis but not sialadenitis in autoimmune-prone BAFF transgenic mice lacking marginal zone B cells.

B cell-activating factor belonging to the TNF family (BAFF) is a B cell survival factor required for B cell maturation. BAFF transgenic (Tg) mice develop autoimmune disorders characterized by autoantibody production, which leads to nephritis and salivary gland destruction (sialadenitis), features reminiscent of systemic lupus erythematosus and Sjögren's syndrome (SS), respectively. Disease in BAFF Tg mice correlates with the expansion of the marginal zone (MZ) B cell compartment and the abnormal presence of MZ-like B cells in the blood, LN and inflamed salivary glands, suggesting a role for these cells in BAFF-induced autoimmunity. Lymphotoxin-beta (LTbeta)-deficient mice show disrupted splenic architecture, lack MZ B cells and some peripheral LN, and are unable to mount T cell-dependent immune responses. BAFF Tg mice lacking LTbeta (LTbetaDelta-BTg) retained these defects, yet still developed nephritis associated with the presence of B-1 B cells in the kidneys. However, in contrast to old BAFF Tg mice, aging LTbetaDelta-BTg mice no longer developed sialadenitis. Thus, autoimmune disorders in BAFF Tg mice are possibly events coordinated by MZ and B-1 B cells at separate anatomical sites.

Expression of lymphotoxin beta governs immunity at two distinct levels.

Interaction of lymphotoxin alpha(1)beta(2) (LTalpha(1)beta(2)) with its receptor is key for the generation and maintenance of secondary lymphoid organ microstructure. We used mice conditionally deficient for LTbeta on different lymphocyte subsets to determine how the LTbeta-dependent lymphoid structure influences immune reactivity. All conditionally LTbeta-deficient mice mounted normal immune responses against vesicular stomatitis virus (VSV), and were protected against lymphocytic choriomeningitis virus (LCMV). In contrast, they exhibited reduced immune responses against non-replicating antigens. Completely LTbeta-deficient mice failed to retain VSV in the marginal zone and died from VSV infections, and they became virus carriers following infection with the non-cytopathic LCMV, which was correlated with defective virus replication in dendritic cells. It was ruled out that LTbeta expression on lymphocytes influenced their activation, homing capacity, or maturation. We therefore conclude that LTbeta expression influences immune reactivity at two distinct levels: (i) Expression of LTbeta on lymphocytes enhances the induction of immune responses against limiting amounts of antigen. (ii) Expression of LTbeta on non-lymphocytes governs antiviral immunity by enhancing antigen presentation on antigen-presenting cells. This prevents cytotoxic T lymphocytes exhaustion or death of the host by uncontrolled virus spread.